The present invention relates to an adaptive envelope extracting apparatus, a signal decoding apparatus and a short-distance contactless communication apparatus applying the adaptive envelope extracting apparatus, and more particularly to an adaptive envelope detector with adaptive threshold decision, and method thereof.
Short-range, standards-based contactless connectivity technology such as Near field communication (NFC) uses magnetic field induction to enable communication between electronic devices in close proximity. Based on RFID technology, NFC provides a medium for the identification protocols that validates secure data transfer. The signal of the NFC is modulated by ASK (Amplitude shift keying) modulation. To receive the data of the NFC signal correctly, the envelope of the NFC signal should first be extracted. In other words, the rising edge and the falling edge of the envelope of the NFC signal should be precisely detected in order to correctly demodulate the data of the NFC signal. For example, an NFC device may refer the length of low voltage level in the envelope to determine the data bit of the NFC signal. If the error of the length of the low voltage level in the envelope is too big, then the data bit of the NFC signal may not be correctly demodulated. In another example, for the NFC PICC devices to be compliant to ISO 14443, the frame delay time (FDT), which is defined as the time between the transmit and receive frames, must be within the specified timing window, i.e. a specific integer multiple of carrier signal (fc) with tight tolerance of 5/fc. Therefore, an accurate detection of the envelope of the NFC signal is required to generate the accurate FDT time.
Conventionally, a rectifier based envelope demodulator is used to extract the envelope of the NFC signal. However, the rectifier based envelope demodulator is not immune to the non-monotonic glitches in the NFC signal. For example, a conventional envelope demodulator is arranged to use a diode to rectify the NFC signal, and use an RC (resistor-capacitor) circuit to output the envelope of the NFC signal. In this conventional envelope demodulator, as shown in FIG. 1, which is a timing diagram illustrating the ripple effect of the conventional envelope demodulator, the ripple effect will appear on the extracted envelope due to capacitor discharging effect for successive peak. The time constant of the RC circuit may cause the negative clipping effect as shown in FIG. 1. The nonlinear behavior of the diode may cause distortion to the envelope as the current flows through the diode. In addition, the RC circuit may occupy a large chip area. Therefore, providing an envelope detector capable of precisely detecting the falling and rising edges of the envelope of the NFC signal is an urgent problem in the NFC field.